1. Field of the Invention
This invention relates to data display arrangements of a type for displaying on the screen of a raster scan display device data represented by digital codes, the displayed data being composed of discrete characters arranged in character rows each comprising a number of character positions.
2. Description of the Prior Art
Data display arrangements of the above type have application in video terminals of a variety of different data display systems for displaying data on the screen of a CRT (cathode ray tube) or other raster scan display device. One such data display system, for instance, is used in conjunction with telephone data services which offer a telephone subscriber having a suitable video display terminal the ability to gain access over the public telephone network to data sources from which data can be selected and transmitted in digitally coded form to the subscriber's premises for display. Examples of this usage are the British and German videotex services Prestel and Bildschirmtext.
A data display arrangement of the above type can include, in addition to the CRT or other display device, acquisition means for acquiring transmission information which represents characters selected for display and also represents size and other attributes for the characters, memory means for storing derived digital codes, and a character memory in which is stored character data identifying the available character shapes which the arrangement can display. This character information is selectively addressed in accordance with the stored digital codes and the information read out is used to produce character generating signals for the data display. Where, as would usually be the case, the display is on the screen of a CRT, this selective addressing is effected synchronously with the scanning action of the CRT.
To facilitate this selective addressing, it is convenient to store the character information as patterns of discrete dots which define the character shapes as corresponding patterns of data bits in respective character memory cell matrices. With this form of storage, the dot pattern of a character shape as displayed on the screen of the CRT can have a one-to-one correspondence with the stored bit pattern for the character. The display may be produced with or without interlaced field scanning.
In order to further facilitate the aforesaid selective addressing, it is also convenient to display characters of a standard size arranged in character rows which can contain up to a fixed maximum possible number of character positions. This standardization determines the size for a rectangular character display area at each position, composed of a plurality of dot rows, which is required for displaying one character. In general, the dot rows are displayed once in successive scanning lines in each field.
With a view to enhancing the display facilities of the data display arrangement, the selective display of characters of normal height, double height, double width and double size has been proposed. For this proposal, a double height character will occupy two corresponding character display areas in adjacent character rows, a double width character will occupy two adjacent character display areas in the same character row, and a double size character will occupy two adjacent character display areas in the same character row and the two corresponding character display areas in an adjacent character row. However, in order to avoid having to store double height, double width and double size bit patterns, it is usual instead to modify the addressing of the existing stored bit patterns for normal height characters.
This modified addressing can be determined by so-called "attribute" logic which causes each bit row of a normal character bit pattern to be read-out an appropriate number of times to expand the character as displayed to the requisite size.
The different size characters, as displayed, may be randomly interleaved. However, because each of the different forms of expanded characters encroach into adjacent display areas which might otherwise be occupied by other characters, certain rules govern the display of expanded characters. These Rules can be:
1. A double height character extends downwards and obscures the character below it. The origin of the character is the upper character position.
2. A double width character extends to the right and obscures the character to its right. The origin of the character is the left hand character position.
3. A double size character extends both downwards and to the right and obscures the three characters, below it, to its right, and to its right and below. The origin of the character is the top left character position.
The effects of these rules are illustrated symbolically in FIG. 1 which will now be considered. In FIG. 1, DBH=double height attribute, DBW=double width attribute, DBS=double size attribute, and NSE=normal size attribute. Sketch (a) illustrates Rule 1. This sketch shows four stored characters A,B,C,D in memory MEM. Character A has the attribute DBH stored with it, so it is displayed double height in display DIS. The character C is obscured in the display, but remains in memory and so will be seen if the attribute DBH is removed. Sketch (b) shows that with the attribute DBW acting on the character A, the character B is obscured, and sketch (c) shows that with the attribute DBS acting on the character A, all the characters B, C and D are obscured.
The obscuring by enlarged characters will also have the effect that if the origin of an enlarged character is obscured, then that character does not display at all. Sketch (d) illustrates this effect; i.e. the double height character B obscures the origin for the character E which would otherwise be displayed double size. Also, sketch (e) illustrates that parts of enlarged characters may not be displayed because of obscuring; i.e. character C is displayed only normal size because of the double height character B. In general, the display priority is in scan-order in that the attribute of the character which is scanned first is considered first.
It has been found that the above rules and their individual effects can give rise to "ripple" effects through the entire display. For instance, if the character A in sketch (d) is given the attribute DBW, as shown in sketch (f), then it can be seen that the entire display is altered. The character B is obscured by the double width character A, so that the origin of the double size character E is no longer obscured by the double height character B and is therefore displayed. It will be appreciated that in an entire display composed of many characters, the overall ripple effect can be considerable. (It should be mentioned that stored characters with no size attribute are displayed normal size. The attribute NSE is provided to revert characters to normal size display following the display in the same character row of an enlarged character).